Silicon carbide is a promising material for future power electronics applications because it can sustain much higher voltages than silicon and has a thermal conductivity similar to copper.
Silicon carbide exists in several different crystal forms (or “polytypes”) depending on the sequence in which bilayers of silicon and carbon stack.
The most commonly-used polytype of silicon carbide is four-step hexagonal stacking sequence silicon carbide (4H-SiC) because it is possible to grow this in single crystal form and produce wafers of the semiconductor material. However, these crystals are produced by physical vapour transport (PVT) process in which a powder of silicon carbide is sublimed at about 2,200° C. producing a vapour which travels and then condenses on a seed crystal. This process is very energy intensive and so silicon carbide wafers are much more expensive to produce than silicon wafers.
Another polytype of silicon carbide, 3-step cubic silicon carbide (3C-SiC), can in principle be grown epitaxially on silicon wafers because they share a cubic crystal form. In this case, a layer of silicon carbide for device fabrication could be realised more cheaply than fabricating a 4H-SiC wafer. However, there are two significant challenges to epitaxially growing a layer of 3-step cubic silicon carbide on silicon, i.e. 3C-SiC/Si heteroepitaxy.
Firstly, there is a lattice mismatch between 3-step cubic silicon carbide and the silicon wafer seed.
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Secondly, silicon carbide and silicon have different coefficients of thermal expansion. When a layer of silicon carbide is grown on silicon at elevated temperatures and then cooled to room temperature, the silicon carbide contracts at a faster rate than silicon, thus the resulting structure bows.
When growing indium gallium aluminium nitride (InGaAlN) on a sapphire or silicon carbide substrate, warping or cracking can be reduced by forming a layer of silicon dioxide on the substrate and selectively growing InGaAlN through openings in the silicon dioxide layer on the exposed parts of the substrate, as described in JP 10 135140 A. InGaAlN is generally grown at temperatures of no more than about 600° C. which is well below the melting point of the substrate. However, silicon carbide is generally grown at temperatures in excess of 1200° C. Furthermore, silicon carbide is grown using gas mixtures which etch silicon dioxide at such high temperatures.
Attempts have been made to address the problem of cracking in layers of silicon carbide by growing silicon carbide on a single-crystal silicon-germanium substrate having a Ge content of between 5 and 20%, as described in WO 03069657 A.